1. Field of the Invention
The present invention generally involves telecommunication systems and more specifically involves telecommunications coupling devices.
2. Description of the Related Art
Modems are commonly used to permit a computer or facsimile machine to communicate with another like device over the telephone line. Conventional modems use a two-wire communications line that is typically connected directly to a two-wire telephone line.
In many telecommunications applications, it is not possible to use a dedicated telephone line for interconnection with a modem or facsimile machine. In telephone systems using a PBX arrangement, where many lines are served by a central controller, it is not always possible to obtain dedicated line service for data communications. In the PBX arrangement, reservation of dedicated telephone lines for data communications may result in under-utilization of the installed capacity.
With a conventional telephone set, the user may need to use a voice capable line for data communication purposes. The modem or facsimile machine may be coupled directly to such a telephone line if the necessary interconnection plug is available. In many situations, coupling of the modem or facsimile machine to the telephone line may be physically difficult, or impossible, without damaging the existing telephone set to telephone line interconnection. For example, many businesses and hotels have installed telephone sets that are hard-wired directly to a corresponding telephone line, having no modular plug and jack interconnection capability. Without the availability of modular jack interconnection to the telephone line, no available means exist for direct connection of the modem or facsimile machine to the telephone line.
In view of this difficulty, practitioners of the art have developed an acoustic coupling technique that is commonly used to couple data communications devices to the telephone line where no direct interconnection means are available. One such commonly used interface couples from the handset of a telephone to a device which translates sound signals emanating from the handset speaker into appropriate signals for use by the modem. This device also translates signals from the modem into an audio signal for detection by the handset microphone. The acoustic coupler device is used by dialing the phone number of a remote auto-answer modem whereupon the remote auto-answer circuitry connects to the telephone line and transmits a carrier signal. Upon hearing this carrier signal, the user places the handset into the acoustic coupling device. A microphone in the acoustic coupling device detects the sound of the carrier from the remote modem through the speaker in the handset. The acoustic coupling device then converts it into a signal which is interpreted by the local modem to mean that a connection has been made. The local modem completes the circuit with the remote modem by sending its own carrier through the acoustic coupling device, by way of the phone line, to the remote modem. Data communication between the two sites begins following this hand-shaking procedure. The reliability of this type of acoustic interface is poor for several reasons. The most significant problem with the acoustic coupling technique is the low signal-to-noise-ratio (SNR) resulting from the repeated conversion of the data signals from electronic form to audio form and back to electronic form again. Noise is introduced at each step of this conversion process, thereby reducing the SNR (signal-to-noise-ratio) of the data signals. These difficulties have for many years limited the application of this type of acoustic coupling device to data transmission rates of approximately 300 baud.
There is a strongly felt need in the community for a device which will permit coupling of modems and facsimile machines to a telephone line, where no direct coupling means are available, which will permit the high speed transmission of data. Such an interconnection capability is required for use with public telephones, car phones, multi-line PBX telephone sets, and hotel telephone sets having no direct interconnection means. The acoustic coupling device described above, known in the present art, is not suitable for these applications because of serious SNR degradation caused by intense levels of ambient noise in locations such as airports, roadside public telephones, automobile telephones during transit, and remotely located telephones facing poor transmission line SNR qualities. The modern transmission of data requires phase-shift modulation (PSM) and frequency-shift modulation (FSM) at data rates from 2,400 baud to 9,600 baud and above. These high-speed modulation-demodulation schemes require SNRs significantly above those available from existing acoustic coupling technology.
Practitioners of the electronic arts have demonstrated substantial interest in developing methods for interconnecting data transmission devices to the telephone lines. U.S. Pat. No. 4,367,374 issued to Arthur L. Serrano on Jan. 4, 1 983, discloses such a method that uses a circuit that directly couples a telephone handset to a modem with provision for monitoring data on the phone line. Serrano's invention requires that the telephone handset be plug-removable from the telephone base unit and is not a workable method for interconnecting at public telephones or mobile phones.
U.S. Pat. No. 4,446,335 issued to Charles E. Leeet al. on May 1, 1984, discloses an audio signal recording apparatus which permits the tapping of audio signals at the plugable interface between a telephone handset and a telephone base unit. Lee et al does not teach a method of interconnecting to telephone lines where telephone handsets are not plug removable from telephone base units. Similarly, U. S. Pat. No. 4,907,267 issued to Howard A. Gutzmer on Mar. 6, 1990, discloses an apparatus for inter-connecting a telecommunications device with the telephone system by way of the plug-removable telephone handset interface. Also, U.S. Pat. No. 4,160,122 issued to Sava W. Jacobson on Jul. 3, 1979, discloses a telephone earphone amplifier device which connects to the telephone base unit through the plug-removable handset interface.
The present invention successfully overcomes the difficulties caused by the SNR deterioration that occurs in acoustically noisy environments when coupling a data modem or facsimile machine to an acoustic telephone handset. The present invention is an unique apparatus for enabling high-speed data communications through a telephone set over a single telephone line where no direct electrical plug-removable interconnection means are available.